Red Sandstone
Silica sand cemented with iron oxide (SiO2 + Fe2O3)
Iron-stained sandstone whose red color comes from hematite coatings, formed in oxidizing desert, river, and coastal environments.
- Mohs hardness
- 6-7
- Color
- Red, reddish-brown, to orange
- Type
- sedimentary
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Overview
Red sandstone is a sedimentary rock made of cemented sand grains, predominantly quartz, that owes its red to reddish-brown color to thin coatings of iron oxide (hematite) on the grains. It belongs to the broad family of 'redbeds,' sediments deposited under oxidizing conditions.
Famous examples include Britain's Old Red Sandstone and New Red Sandstone and the red rock formations of the American Southwest. Red sandstone is a classic building stone and a striking landscape former, carving into mesas, arches, and cliffs.
Formation & geology
Red sandstone forms when sand accumulates in environments where iron is oxidized, such as deserts, river floodplains, deltas, and coastal plains under a warm, dry climate. As the sand is buried and cemented by silica, iron oxide, or carbonate, hematite forms a thin red film coating the grains.
The red color typically develops through oxidation during or after deposition rather than indicating a high iron content. Cross-bedding, ripple marks, and desiccation cracks are common, recording windblown dunes or flowing water. Many red sandstones formed in ancient arid continental basins.
How to identify it
Look for a sand-grained, gritty rock in shades of red, reddish-brown, or orange. The grains, mostly quartz, are visible and give a hardness near 6-7, so the rock scratches glass, though weakly cemented examples crumble easily.
The rock does not fizz in acid unless it has carbonate cement. Sedimentary structures like cross-bedding and ripple marks are often visible. Streak is generally white from the quartz, with reddish dust from the iron coating.
Look-alikes include arkose (rich in feldspar grains) and ironstone (much heavier, iron-dominated). Red sandstone is distinguished by its quartz sand framework with only a thin iron-oxide stain, not a true iron-rich composition.
Uses & significance
Red sandstone is a popular building and dimension stone, used historically in cathedrals, bridges, and 'brownstone' townhouses, and today in facing, paving, and landscaping. It is relatively easy to quarry and carve yet durable.
It is also crushed for aggregate and used as a raw material in some industrial processes. Red sandstone formations are important petroleum and groundwater reservoirs due to their porosity. Their dramatic scenery in places like the American Southwest also makes them major tourist and recreation landscapes.
Frequently asked questions
Why is red sandstone red?
Thin coatings of iron oxide (hematite) on the sand grains give it red and reddish-brown colors, formed under oxidizing conditions.
Is red sandstone good for building?
Yes. It is widely used as dimension and building stone because it is easy to carve yet durable, as in brownstone houses.
Does red sandstone contain a lot of iron?
Not usually. The red comes from a thin hematite stain on quartz grains, not a high overall iron content.
Where is red sandstone found?
Famous examples include Britain's Old and New Red Sandstones and the red rock country of the American Southwest.
Red Sandstone guides
In-depth guides for identifying, valuing, and understanding Red Sandstone.
Red Sandstone identified by the community
Real specimens identified with Rock Identifier.
